is good, RO
water that is then filtered through DI
resin, making it even better. If you're into reef tanks for the long haul, you'll save a ton of money by getting a RO
filter and making it yourself. airwaterice.com has some great ones, as well as you can find them on ebay.
Like Tank said, man I'd go slow in that small of a system. If you overshoot, the only way to get it back down is to remove SW
from the tank and replace with clean water. Not a big deal, but it's just a lot easier to slowly raise it to the level you want it. I also find that with reef crystals that their recommended amounts are too high. I'm still a newb, but IIRC they say 1/2 cup for 1g of clean water. I find that to raise the sg
too high, so it's somewhere less than that.